When metal components are used for vessels and piping in chemical processing and for structural purposes, it is important to be aware of locations that may be corroding and losing the capabilities for which they were designed. Localized corrosion is most critical in this regard.
Localized corrosion is distinct from general corrosion. As general corrosion takes place, relatively large areas of the surface corrode together, at about a uniform rate. An important kind of localized corrosion is pitting corrosion, which occurs initially in a microscopically tiny area on a material surface and then gets larger and deeper, forming pits in the surface. Electrochemically the area affected becomes consistently anodic in the corrosive medium. Pitting phenomena can be erratic: often a localized anodic reaction starts up and shuts down because of concentration fluctuations in the corrosive medium (or other reasons) and formation of a passive layer on the metal (known as repassivation of the metal surface); such action is known as metastable corrosion and is a precursor of the more damaging fully developed pitting corrosion.
Localized corrosion, particularly pitting, is insidious because material is removed in a concentrated small area that is not easily recognized. One of the most dangerous consequences of pitting corrosion is a leak in a containment vessel such as a tank or pipeline. The leak typically occurs at a pinhole in a wall of the containment vessel. The majority of the wall still has adequate thickness to contain the vessels contents. The resulting leak can be especially dangerous where the contained material is under pressure, at high temperature, or both. The present invention provides a warning system in real time, making the user aware of the extent of localized corrosion before the structure develops pinholes. In extreme cases, pitting can lower the structural integrity of a structure, resulting in a mechanical failure.
Several methods for monitoring localized corrosion are based on electrochemical noise analysis. See U.S. Pat. Nos. 4,575,678 to Hladkey and 5,139,627 to Eden, et al., the disclosures of which are incorporated herein by reference. In the Hladkey patent, fluctuations in the potential between two electrodes are related to corrosion processes. In the Eden, et. al. Patent, a Zero Resistance Ammeter (ZRA) is used to measure current fluctuations between two similar metal electrodes. The ZRA works to match the voltages of the two electrodes while it measures current flow between them. Any current fluctuations are due to "noise" or small statistical variations on the specimen surface. In addition, an electrometer is used to measure potential fluctuations between one of the probes and a reference electrode. The measured values are then filtered and ratioed to produce several reported numbers including a "Noise Resistance" and a "Pitting Index." Any variations are due to localized corrosion since the metal electrodes, being in the same environment at the same electrochemical potential, should have the same general corrosion rate.
A problem with the aforesaid conventional method is that not all of the localized corrosion currents are measured by the ZRA. Usually much of the anodic current originating at the pitting site will be sunk cathodically on the same electrode, never appearing in the ZRA measurement circuit. The present invention provides a means for localizing the reactions on the electrodes, with most of the cathodic reactions occurring on one electrode and the anodic reactions on another. The current measured by the circuit is therefore both larger and a much better measure of the localized corrosion current.
Because the pitting effect itself is localized, the total signal generated by it is small and difficult to detect. To add to the difficulties of measurement, various kinds of electrical noise in or near the structure effectively mask and distort the comparatively weak electric signals that are characteristic of pitting noise. Motors, circuit breakers, switches and radio frequency generators all add to the cacophony tending to obscure the corrosion noise signal. The present invention, with its larger currents, is less susceptible to interference from these external noise sources.
A further problem is that the signal indicative of pitting is defined in the Eden et al. patent as I.sub.noise /I.sub.mean. In this definition I.sub.mean can vary with the general corrosion rate. Indeed, if the ZRA is working as designed and the chemical environments of both electrodes are the same, the expected value of I.sub.mean is 0, which leads to a measurement singularity. In addition the aforesaid involves extensive interpretation of values, in order to identify the type and rate of corrosion.